1. Field of the Invention
The present invention relates generally to determining geological properties of subsurface formations using nuclear magnetic resonance (NMR) measurements for logging boreholes and, particularly, to calibrating apparatus used to perform the measurements.
2. Description of the Related Art
Boreholes are drilled into the earth for many applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. In order to efficiently use expensive resources drilling the boreholes, it is important for analysts to acquire detailed information related to the geologic formations being drilled.
Nuclear magnetic resonance (NMR) tools are one type of downhole tool that is particularly useful for performing detailed measurements of properties of hydrocarbon bearing formations. The NMR measurements are used to determine among other things, porosity, hydrocarbon saturation, and permeability of rock formations. The NMR logging tools are used to excite the nuclei of the fluids in the geological formations surrounding the borehole so that certain nuclear spin parameters such as nuclear spin density, longitudinal relaxation time (generally referred to in the art as T1) or transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, the porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A voltage is induced in a receiving coil disposed at the NMR tool by nuclear spins that precess in response to a sequence of magnetic field intensities applied to the nuclei. The induced voltage is typically very small and an amplifier is used to amplify the induced voltage. Because the induced voltage can be very small, various factors can distort and reduce the accuracy of the measurements. These factors include changes in the electrical conductivity of the drilling mud, changes in the electrical conductivity of the formation, and changes in the diameter of the borehole. It would be well received in the drilling industry if the accuracy of NMR measurements could be improved, when borehole and formation parameters may be changing fast.